Programmable storage arrays adapted to store binary information for subsequent readout are known from the prior art. The prior art arrays such as shown in U.S. Pat. No. 3,699,543 to Neale which issued October 17, 1972 have included X and Y sets of address lines insulated from one another and oriented to be orthogonal to one another so as to form a plurality of cross-over points where the address lines cross one another. It has also been known from the prior art at each of the cross-over points to form an electrical isolation element such as a diode, in series with a settable storage element. This series combination is then connected between the corresponding X address line and the corresponding Y address line to form the addressable memory cell.
Such arrays are also referred to in U.S. Pat. No. 4,203,123 to Shanks which issued May 30, 1980. Shanks refers to forming the isolation diodes from amorphous silicon materials. Such diodes are disclosed in U.S. Pat. No. 4,226,898 to Ovshinsky which issued October 7, 1980. The '898 patent discloses both PN junction diodes and PIN type diodes.
Further it has been known from the prior art as in Shanks to use as the memory elements amorphous chalcogenide materials which can be set or reset so as to have a high or a low conductivity. In the prior art as shown in Shanks it has been known to form the amorphous silicon diodes physically above or below the chalcogenide based memory element. Neale has disclosed off-setting the diodes with respect to the settable memory elements.
In addition to being able to fabricate a usable array it's necessary to set selected storage locations to a condition corresponding a 1 or a 0, either during or after the fabrication process. When a read only memory is to be provided the programming can take place during the manufacturing process. In the case of reprogrammable memory arrays the programming may take place once the array has been fabricated.
Some of the problems associated with the prior art derive from the vertical arrangement of the isolation diode and memory cell assembly at the cross-over areas. In this configuration the resistance of the cell in series with the isolation diode is directly proportional to the thickness of the chalcogenide material. Therefore a low thickness of the chalcogenide film could result in an inadequately low value of the cell's "off" resistance.
A second serious problem is the deleterious influence of dust grains that could create a high concentration of point defects. When the film cell is sandwiched between two conductors grains of dust can create electrical shorts shunting the cell's impedance. The third serious problem is associated with the use of a transparent conductive electrode fabricated over the memory film to make it possible to optically program the array. The fabrication process of this transparent electrode involves heat treatments often harmful to the properties of the memory film. Moreover the sheet resistivity of the conductive transparent electrode is expected to be one order of magnitude larger than an ordinary metal electrode. This higher sheet resistance can have an adverse effect on the impedance of the address lines.
In the past the vast majority of integrated circuits have been formed on crystalline substrates which are usually quite rigid and brittle, requiring that such substrates be mounted on relatively thick, flat surfaces in order not to be broken. There are, however, applications in which it would be desirable to have integrated circuits which could be mounted on flexible surfaces, such as the surfaces of letters, packages or other objects which are not flat. The advantage of forming circuits upon such flexible substrates grows as does the size of the circuits involved, since large crystalline integrated circuits are even more subject to breakage than are small ones.
There has been some use of flexible substrates as surfaces on which to form circuit elements. For example, flexible substrates have been formed of synthetic polymeric resins, such as the high temperature polyimide sold under the trademark "Kapton" by the Dupont Company, Polymeric Products Department, Industrial Films Division, Wilmington, Del. 19898. When used as an electronic substrate, Kapton can withstand temperatures of up to 300.degree. centigrade and the material has been used widely as a flexible substrate upon which metal lines are formed by photolithographic techniques and then upon which integrated circuits are mounted by soldering.